⚗️ Water alkalinity
In short
Titrimetric determination of total alkalinity and phenolphthalein alkalinity in natural and treated waters and in waste water with hydrochloric acid, the end point being detected with a pH meter or with indicators. The test is performed according to PN-EN ISO 9963-1:2001; Range: up to 20 mmol/l without dilution; recommended lower limit 0.4 mmol/l. The procedure comprises 5 steps; it is used for: Natural and treated waters and waste water (clause 1), Monitoring of surface and ground waters — total alkalinity (Journal of Laws 2021, item 1576), Distinguishing total alkalinity (pH 4.5) and phenolphthalein alkalinity (pH 8.3) (clause 3).
At a glance
- Standard: PN-EN ISO 9963-1:2001
- Category: Physicochemistry
- Procedure steps: 5
- Range: up to 20 mmol/l without dilution; recommended lower limit 0.4 mmol/l
- End points: pH 8.3 (phenolphthalein alkalinity) and pH 4.5 (total alkalinity)
- Titrant: HCl 0.1 mol/l for 4–20 mmol/l; HCl 0.02 mol/l for 0.4–4 mmol/l
Overview
STATUS (as of 29.10.2026). PN-EN ISO 9963-1:2001 — Polish version, published 23.04.2001, implements EN ISO 9963-1:1995 and ISO 9963-1:1994 [IDT]; it replaced PN-C-04540-02:1990 and PN-C-04540-03:1990. The standard is current — no withdrawal note; it has the national amendment PN-EN ISO 9963-1:2001/Ap1:2004. ISO 9963-1:1994 and EN ISO 9963-1:1995 are current in the ISO and CEN catalogues, with no newer edition.
NATIONAL AMENDMENT Ap1:2004 (approved 8.06.2004). It changes the formula for the hydrochloric acid concentration in 5.3.4 to c(HCl) = m · V₁ / [53.00 · (V₂ − V₃)] — we give it below in this form (it agrees with the ISO text).
SCOPE (clause 1). Titrimetric determination of the alkalinity of natural and treated waters and of waste water; directly up to 20 mmol/l, at higher concentrations a smaller test portion; recommended lower limit 0.4 mmol/l. Suspended matter in the form of carbonate may interfere — filtration before the titration reduces its effect. End-point detection with a pH meter is less prone to interferences than with an indicator; strongly coloured or turbid samples are analysed by the potentiometric method (clause 8).
DEFINITIONS (clause 3). Alkalinity — the quantitative capacity of aqueous media to react with hydrogen ions. Total alkalinity (AT) — an arbitrary measurement obtained by titration to the methyl red (methyl orange) end point, pH 4.5; phenolphthalein (composite) alkalinity (AP) — titration to pH 8.3, arbitrarily attributed to all the hydroxide and half the carbonate. Alkalinity is primarily a function of hydrogen carbonate, carbonate and hydroxide, but other buffering substances such as ammonia, borate, phosphate, silicate and organic anions may be included in the determination. By definition, the phenolphthalein alkalinity of waters with a pH of 8.3 or less is zero.
ALKALINITY IN WATER. Natural alkalinity protects aquatic organisms against sudden acidification and takes part in neutralizing waste water, especially acidic waste water, discharged to waters; it is also a technological parameter of some water treatment processes, e.g. coagulation.
LEGAL REQUIREMENTS. In the monitoring of surface and ground waters, PN-EN ISO 9963-1 is the titrimetric method for total alkalinity and, with amendment Ap1:2004, the potentiometric titration method (Journal of Laws 2021, item 1576, Annex 8, indicator 3.4.2). The regulation on water intended for human consumption (Journal of Laws 2026, item 748) has no alkalinity parameter.
Method principle
The sample is titrated with a standard hydrochloric acid solution to fixed end points of pH 8.3 and 4.5, detected visually or potentiometrically. These are the selected equivalence points for the three principal components — hydrogen carbonate, carbonate and hydroxide: pH 8.3 approximates the equivalent concentrations of carbonate and carbon dioxide (titration of approximately all the hydroxide and half of the carbonate), and pH 4.5 approximates the equivalence point for hydrogen ion and hydrogen carbonate, which allows total alkalinity to be determined. The actual equivalence points depend on the ionic strength and the total inorganic carbon concentration. Different indicator systems give slightly different results — according to the standard, the mixed bromocresol green–methyl red indicator is used, not methyl orange or methyl red, which were used in the past (clause 4, note 2).
Applications
- Natural and treated waters and waste water (clause 1)
- Monitoring of surface and ground waters — total alkalinity (Journal of Laws 2021, item 1576)
- Distinguishing total alkalinity (pH 4.5) and phenolphthalein alkalinity (pH 8.3) (clause 3)
- Strongly coloured and turbid samples — potentiometric procedure (clause 8)
Key parameters
| Parameter | Value |
|---|---|
| Range | up to 20 mmol/l without dilution; recommended lower limit 0.4 mmol/l |
| End points | pH 8.3 (phenolphthalein alkalinity) and pH 4.5 (total alkalinity) |
| Titrant | HCl 0.1 mol/l for 4–20 mmol/l; HCl 0.02 mol/l for 0.4–4 mmol/l |
| Test portion | 100 ml |
| pH meter | accuracy ± 0.05 pH units over the range 4–10; calibration preferably with buffers of pH 4, 7 and 9 |
| Result | in mmol/l (capacity to react with hydrogen ions) |
Standard
- Standard number
- PN-EN ISO 9963-1:2001
- Title (PL)
- Jakość wody — Oznaczanie zasadowości — Część 1: Oznaczanie zasadowości ogólnej i zasadowości wobec fenoloftaleiny
- Title (EN)
- Water quality — Determination of alkalinity — Part 1: Determination of total and composite alkalinity
Step-by-step procedure
-
Sampling
In clean polyethylene or borosilicate glass bottles of at least 100 ml, filled completely and stoppered so that no air remains inside. Ideally analyse immediately after collection; if this is not possible, store in a cool place to avoid nitrification or scaling. The effect of storage is tested for the type of samples analysed (clause 7).
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Potentiometric — to pH 8.3
Calibrate the pH meter. Put 100 ml of sample in the titration vessel, stir at a rate at which a vortex is just not perceptible and measure the pH: if it is 8.3 or less, the phenolphthalein alkalinity is zero. Acid 0.1 mol/l for alkalinity of 4–20 mmol/l, 0.02 mol/l for 0.4–4 mmol/l; titrate to pH 8.3 (volume V₅). Absorption of atmospheric carbon dioxide can lower this result (8.1.1, note 4).
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Potentiometric — to pH 4.5
Continue titrating the same solution to pH 4.5 ± 0.05; near pH 4.5 add the titrant drop by drop and allow at least 30 s for the electrodes to reach equilibrium; total volume of acid — V₆. Organic substances, soaps and oily substances may coat the glass electrode and slow its response — then allow more time between titrant additions and clean the electrodes frequently (8.1.2, note 3).
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Visual procedure
Free chlorine is removed with 0.1 ml of thiosulfate solution per 200 ml of sample (up to 1.8 mg/l of chlorine). 100 ml of sample in a 250 ml flask with 0.1 ml of phenolphthalein: no pink colour — phenolphthalein alkalinity zero; titrate a pink sample with acid until the colour disappears (V₅). Then add 0.1 ml of mixed indicator and continue titrating until the colour changes from greenish-blue to grey (total volume V₆) (8.2).
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Calculation
AP = c(HCl) · V₅ · 1000 / V₄ and AT = c(HCl) · V₆ · 1000 / V₄ [mmol/l], where V₄ is the volume of the test portion (normally 100 ml). Results are expressed in millimoles (H⁺) per litre; conversion to mg/l CaCO₃ — factor 50, to mg/l HCO₃⁻ — factor 61 (clauses 9–10, Annex A).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Magnetic stirrer | with a plastics-coated stirring bar; stirring rate at which a vortex is just not perceptible (6.1, 8.1.1) | — |
| pH meter with electrodes | measuring pH to within ± 0.05 units over the range 4–10, calibrated preferably with buffers of pH 4, 7 and 9 (IEC 746-2); titration vessel designed so that contact with air is minimized (6.2) | — |
| Precision burette, 10 ml | graduated in divisions of 0.02 ml, conforming to ISO 385-1 (6.3) | — |
| Conical flasks, 250 ml | for the visual procedure (8.2) | — |
| Sample bottles | clean polyethylene or borosilicate glass bottles of at least 100 ml (clause 7) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Water | — | grade 2 according to ISO 3696, free of interfering concentrations of acid or alkali, with a conductivity of less than 0.1 mS/m (5.1) |
| Sodium carbonate standard solution, about 0.025 mol/l | 497-19-8 | dry 3–5 g of Na₂CO₃ at (250 ± 10) °C for 4 h and cool in a desiccator; dissolve (2.65 ± 0.20) g, weighed to the nearest 0.001 g, in water and dilute to 1000 ml; stable for at least one month in a refrigerator at 4–8 °C (5.2) |
| Hydrochloric acid, c(HCl) = 0.10 mol/l | 7647-01-0 | dilute (8.6 ± 0.1) ml of hydrochloric acid (1.16 g/ml) to 1000 ml; standardize on 25.0 ml of sodium carbonate solution with 75 ml of water — potentiometrically to pH 4.5 ± 0.05 or visually with 0.1 ml of mixed indicator until the greenish-blue colour disappears, with a blank on 100 ml of water: c = m · V₁ / [53.00 · (V₂ − V₃)]; standardize at least weekly (5.3) |
| Hydrochloric acid, c(HCl) = 0.02 mol/l | — | dilute 100 ml of 0.10 mol/l acid to 500 ml; prepare freshly before use; c₂ = 0.2 · c₁ (5.4) |
| Phenolphthalein indicator solution | 77-09-8 | dissolve (1.0 ± 0.1) g in 100 ml of ethanol (more than 90 % by volume) and dilute with water to 200 ml (5.5) |
| Bromocresol green–methyl red mixed indicator | — | dissolve (0.200 ± 0.005) g of bromocresol green and (0.015 ± 0.002) g of methyl red in 100 ml of ethanol (more than 90 % by volume); store in an amber glass bottle (5.6) |
| Sodium thiosulfate, 0.1 mol/l | — | (2.5 ± 0.2) g of sodium thiosulfate pentahydrate in 100 ml of water; amber glass bottle, refrigerator, maximum 6 months; for removing free chlorine (5.7) |
Health and safety (OHS)
- The standard gives no safety warnings — handle hydrochloric acid, the ethanolic indicator solutions and sodium thiosulfate in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 9963-1:2001 — “Water quality — Determination of alkalinity — Part 1: Determination of total and composite alkalinity”.
How does this method work?
The sample is titrated with a standard hydrochloric acid solution to fixed end points of pH 8.3 and 4.
What is the measuring range and accuracy?
Range: up to 20 mmol/l without dilution; recommended lower limit 0.4 mmol/l; End points: pH 8.3 (phenolphthalein alkalinity) and pH 4.5 (total alkalinity); Titrant: HCl 0.1 mol/l for 4–20 mmol/l; HCl 0.02 mol/l for 0.4–4 mmol/l; Test portion: 100 ml.
How long does the test take?
The procedure comprises 5 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Magnetic stirrer, pH meter with electrodes, Precision burette, 10 ml, Conical flasks, 250 ml, Sample bottles.
Where is this test used?
Natural and treated waters and waste water (clause 1); Monitoring of surface and ground waters — total alkalinity (Journal of Laws 2021, item 1576); Distinguishing total alkalinity (pH 4.5) and phenolphthalein alkalinity (pH 8.3) (clause 3); Strongly coloured and turbid samples — potentiometric procedure (clause 8).
What safety precautions apply?
The standard gives no safety warnings — handle hydrochloric acid, the ethanolic indicator solutions and sodium thiosulfate in accordance with their safety data sheets.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 9963-1.